The significant polarization components of a medium produced by contact with an electric field are first order polarization (linear polarization), second order polarization (first nonlinear polarization), and third order polarization (second nonlinear polarization). On a molecular level this can be expressed by Equation 1: EQU P=.alpha.E+.beta.E.sup.2 +.gamma.E.sup.3 . . . (1)
where
P is the total induced polarization, PA0 E is the local electric field created by electromagnetic radiation, and PA0 .alpha., .beta., and .gamma. are the first, second, and third order polarizabilities, each of which is a function of molecular properties. PA0 .beta. and .gamma. are also referred to as first and second hyperpolarizabilities, respectively. The molecular level terms of Equation 1 are first order or linear polarization .alpha.E, second order or first nonlinear polarization .beta.E.sup.2, and third order or second nonlinear polarization .gamma.E.sup.3. PA0 P is the total induced polarization, PA0 E is the local electric field created by electromagnetic radiation, and PA0 .chi..sup.(1), .chi..sup.(2), and .chi..sup.(3) are the first, second, and third order polarization susceptibilities of the electromagnetic wave transmission medium. .chi..sup.(2) and .chi..sup.(3) are also referred to as the first and second nonlinear polarization susceptibilities, respectively, of the transmission medium. The macromolecular level terms of Equation 2 are first order or linear polarization .chi..sup.(1) E, second order or first nonlinear polarization .chi..sup.(2) E.sup.2, and third order or second nonlinear polarization .chi..sup.(3) E.sup.3.
On a macromolecular level corresponding relationships can be expressed by Equation 2: EQU P=.chi..sup.(1) E+.chi..sup.(2) E.sup.2 +.chi..sup.(3) E.sup.3. . . (2)
where
To achieve on a macromolecular level second order polarization, .chi..sup.(2) E.sup.2, of any significant magnitude, it is essential that the transmission medium exhibit second order (first nonlinear) polarization susceptibilities, .chi..sup.(2), greater than 10.sup.-9 electrostatic units (esu). To realize such values of .chi..sup.(2) it is necessary that the first hyperpolarizability .beta. be greater than 10.sup.-30 esu.
A significant difficulty encountered in finding materials exhibiting usefully large second order polarization effects lies in the molecular requirements that must be satisfied to achieve usefully large values of .beta.. For a molecule to exhibit values of .beta. greater than zero, it is necessary that the molecule be asymmetrical about its center--that is, noncentrosymmetric. Further, the molecule must be capable of oscillating (i.e., resonating) between an excited state and a ground state differing in polarity. It has been observed experimentally and explained by theory that large .beta. values are the result of large differences between ground and excited state dipole moments as well as large oscillator strengths (i.e., large charge transfer resonance efficiencies). Materials having usefully large values of .beta. are commonly referred to as molecular dipoles.
For .chi..sup.(2) to exhibit a usefully large value it is not only necessary that .beta. be large, but, in addition, the molecular dipoles must be aligned so as to lack inversion symmetry. The largest values of .chi..sup.(2) are realized when the molecular dipoles are arranged in polar alignment--e.g., the alignment obtained when molecular dipoles are placed in an electric field.
For a number of years the materials employed for achieving second order polarization effects were noncentrosymmetric inorganic crystals, such as potassium dihydrogen phosphate and lithium niobate. D. J. Williams, "Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities", Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703, postulated mathematically and experimentally corroborated second order polarizabilities in organic molecular dipoles equalling and exceeding those of inorganic crystals. Electrical poling and Langmuir-Blodgett construction techniques were recognized from the outset to be feasible approaches for polar alignment of the organic molecular dipoles to translate molecular second order polarizabilities into layer second order polarization susceptibilities. Zyss, "Nonlinear Organic Materials for Integrated Optics", Journal of Molecular Electronics, Vol. 1, pp. 25-45, 1985, is essentially cumulative with Williams, surveying applications for organic molecular dipoles to varied nonlinear optical needs.
Garito U.S. Pat. No. 4,431,263; Girling, Cade, Kolinsky, and Montgomery, "Observation of Second Harmonic Generation from a Langmuir-Blodgett Monolayer of a Merocyanine Dye," Electronics Letters, Vol. 21, No. 5, 2/28/85; Neal, Petty, Roberts, Ahmad, and Feast, "Second Harmonic Generation from LB Superlattices Containing two Active Components," Electronics Letters, Vol. 22, No. 9, 4/24/86; and Ulman et al U.S. Pat. No. 4,792,208 provide illustrations of organic molecular dipoles deposited by Langmuir-Blodgett techniques to form layers exhibiting significant .chi..sup.(2) values.
Williams and Zyss are extrapolations from limited demonstrated capabilities to theoretically possible applications. Garito, Girling et al, Neal et al, and Ulman are concerned with Langmuir-Blodgett components to meet device requirements. Electronic and Photonic Applications of Polymers, M. J. Bowden and S. R. Turner Ed., Chapter 6, Polymers in Nonlinear Optics, by D. Williams, American Chemical Society 1988, suggests that polymeric Langmuir-Blodgett films have the theoretical capability of producing useful devices.
What has been accomplished by those skilled in the art is to ascertain that of varied routes available for constructing optical articles exhibiting high second order polarization susceptibilities the use of organic layer units constructed by Langmuir-Blodgett techniques is a theoretically feasible possibility.
What the art has failed to accomplish is the construction by Langmuir-Blodgett techniques of high second order polarization susceptibility optical articles exhibiting sufficiently high levels of stability and low levels of internal optical attenuation to achieve sought after device performance levels.
Penner et al U.S. Ser. No. 07/760,436, concurrently filed and commonly assigned, titled IMPROVED CONVERSION EFFICIENCY SECOND HARMONIC GENERATOR, discloses an optical article comprised of a support including a portion adjacent one major surface which is transparent to the electromagnetic radiation sought to be propagated, an organic layer unit capable of converting a portion of polarized electromagnetic radiation of a selected wavelength to its second harmonic wavelength, means for optically coupling into said organic layer unit polarized electromagnetic radiation of a selected wavelength in its zero order transverse magnetic mode, and means for receiving from the layer unit a portion of the electromagnetic radiation in the form of a first order transverse magnetic mode. The organic layer unit has a thickness which is at least 70 percent of the wavelength of the zero order transverse magnetic mode and differs by less than 100 .ANG. from the thickness required for identical propagation constants of the zero and first order transverse magnetic modes. The organic layer unit is comprised of a Y-type Langmuir-Blodgett assembly of amphiphiles forming a first Langmuir-Blodgett layer unit containing noncentro-symmetric organic molecular dipoles of a first orientation providing a second order polarization susceptibility to the first layer unit in excess of 10.sup.-9 electrostatic units, and a Y-type Langmuir-Blodgett assembly of amphiphiles forming a second Langmuir-Blodgett layer unit adapted to be coated on the first Langmuir-Blodgett layer unit containing noncentro-symmetric organic molecular dipoles of a second orientation providing a second order polarization susceptibility to the second layer unit in excess of 10.sup.-9 electrostatic units, but of opposite sign to that of the first layer unit.